Chapter 23

Heavy-Duty

Truck Axles

Objectives (1 of 3)

• Identify the types of axles used on trucks and trailers.

• Define the terms dead axle, live axle, pusher axle, and tag axle.

• Outline the construction of a drive axle carrier assembly.

• Explain how a pinion and crown gearset change the direction of powerflow.

Objectives (2 of 3)

• Describe differential action and list the reasons it is required.

• Identify the components required to create differential action.

• Describe the operation of the various drive axle configurations.

• Identify the components used in an interaxle differential or power divider.

Objectives (3 of 3)

• Explain how an interaxle differential lock functions.

• Define the term spinout and explain how it is caused.

• Trace the powerflow path through different types of differential carriers.

Examples of Heavy-duty Axles

Typical Steering Axle

Trailer Axle

Single Rear Drive Axle

Cutaway Differential Carrier

Shop Talk

• Each OEM tends to use different words to describe the same components, but in this text we will try to use the most common term.

• A crown gear is also a ring gear, but we usually will use the first term.

• An axle shaft also can be known as a half-shaft or drive shaft; once again, we usually will use the first term.

• A drive shaft connects a drive wheel to a differential, whereas a driveshaft is a propeller shaft that delivers input torque to the final drive carrier. Confused? You bet, but, nevertheless, you should try to become familiar with all the OEM terms so that you can accurately interpret service literature.

Typical Differential Components

Differential Action

Tandem Drive Axles

Final Drive Axle Configurations

• A single-reduction axle

• A planetary double-reduction axle

• A two-speed axle assembly

• A tandem drive axle

Bevel Gears

Hypoid, Amboid, and Spiral Bevel Gears

Single-reduction Carrier

Differential Lock

Hypoid Double-reduction Carrier

Planetary Double-reduction Gearing

Forward Tandem Drive Axle

Torque Distribution with Power Divider Unlocked

Torque Distribution with Power Divider Locked

Power Divider with External Lube Pump

Disengaged Power Divider

Torque Distribution with Normal Traction Conditions

Power Divider Engaged

Torque Distribution When One Axle Loses Traction

Drive Axle ShaftConfigurations

• Two drive axle shaft configurations are used to provide support between the axle hub and the vehicle wheels.– Semi-floating axle shaft– Full floating axle shaft

Non-driving Axles

• There are three categories of non-driving or dead axles.– Steering axles– Lift and tag axles– Trailer axles

Summary (1 of 6)

• Axles can be divided into live axles and dead axles.– Live axles are drive axles; they house the

gearing required to change the direction of powerflow and produce differential action.

– Dead axles are designed to support chassis loads. Dead axles include steering axles, tag axles, pusher axles, and various types of trailer axles.

Summary (2 of 6)

• The drive axles in trucks are known as differential carriers and are mounted in a banjo housing.

• Driveshafts deliver driveline torque to the differential carrier that uses pinion and crown gearing to change the direction of powerflow and transmit it to the drive wheels.

• Differential gearing is required in drive axles and enables an outboard wheel to turn faster than an inboard wheel through a turn.

Summary (3 of 6)

• The critical differential components are a four shaft spider with pinion gears on each shaft that mesh to side gears on either side. The side gears are splined to the axle shaft responsible for driving the wheels.

• During differential action, the slowdown of one side gear is inversely proportional to the increase in speed of its opposite side gear.

Summary (4 of 6)

• Most single-reduction axles are splash-lubricated by gear oil as the geared components rotate, but many differential carriers with power dividers and multiple- reduction gearing use oil pumps to help lubricate the assembly.

• Tandem drive trucks require an inter-axle differential or power divider to divide driveline torque between the two drive axles.

Summary (5 of 6)

• In a tandem drive axle arrangement, differential action in the differential carriers and power divider can cause spinout, in which a one-wheel spin can produce fourfold velocities.

• Most inter-axle differentials can be temporarily locked out by a driver-activated switch. – This can help provide traction in slippery conditions

and reduce the potential for spinout.

Summary (6 of 6)

• Two-speed differential carrier gearing can be used on both single and tandem drive axle vehicles; control is by the driver.

• A double-reduction differential carrier can produce two ratio reductions. The first reduction takes place at the crown and pinion gearing, the second at a planetary gearset.

• Torque proportioning power dividers reduce spinouts by biasing torque transfer to the drive axle with the most traction.